1. Field of the Invention
The present invention is directed to a shape selective hydrocarbon conversion process over a modified catalyst. The invention is also directed to the modified catalyst and method for modifying the catalyst by pre-selectivating with first silicon source followed by steam treatment.
2. Description of the Prior Art
The term shape-selective catalysis describes unexpected catalytic selectivities in zeolites. The principles behind shape selective catalysis have been reviewed extensively, e.g. by N. Y. Chen, W. E. Garwood and F. G. Dwyer, "Shape Selective Catalysis in Industrial Applications, 36, Marcel Dekker, Inc. (1989). Within a zeolite pore, hydrocarbon conversion reactions such as paraffin isomerization, olefin skeletal or double bond isomerization, oligomerization and aromatic disproportionation, alkylation or transalkylation reactions are governed by constraints imposed by the channel size. Reactant selectivity occurs when a fraction of the feedstock is too large to enter the zeolite pores to react; while product selectivity occurs when some of the products cannot leave the zeolite channels. Product distributions can also be altered by transition state selectivity in which certain reactions cannot occur because the reaction transition state is too large to form within the zeolite pores or cages. A final type of selectivity results from configurational diffusion where the dimensions of the molecule approach that of the zeolite pore system. A small change in dimensions of the molecule or the zeolite pore can result in large diffusion changes leading to different product distributions. This type of shape selective catalysis is demonstrated, for example, in toluene selective disproportionation to p-xylene. The production of para-xylene is typically performed by methylation or disproportionation of toluene over a catalyst under conversion conditions. Examples are the reaction of toluene with methanol as described by Chen et al., J. Amer. Chem. Sec. 1979, 101, 6783, and toluene disproportionation, as described by Pines in "The Chemistry of Catalytic Hydrocarbon Conversions", Academic Press, N.Y , 1981, p. 72. Such methods typically result in the production of a mixture including para-xylene, ortho-xylene, and meta-xylene. Depending upon the para-selectivity of the catalyst and the reaction conditions, different percentages of para-xylene are obtained. The yield, i.e., the amount of feedstock actually converted to xylene, is also affected by the catalyst and the reaction conditions. The equilibrium reaction for the conversion of toluene to xylene and benzene proceeds as follows: ##STR1##
One method for increasing para-selectivity of zeolite catalysts is to modify the catalyst by treatment with a "selectivating agents". For example, U.S. Pat. Nos. 4,950,835, 4,927,979, 4,465,886, 4,477,583, 4,379,761, 4,145,315, 4,127,616, 4,100,215, 4,090,981, 4,060,568 and 3,698,157 disclose specific methods for contacting a catalyst with a modifying compound containing silicon.
U.S. Pat. No. 4,548,914 describes another modification method involving impregnating catalysts with difficultly reducible oxides such as those of magnesium, calcium and/or phosphorus followed by treatment with water vapor to improve paraselectivity.
Steaming has been used in the preparation of zeolite catalysts to modify the alpha or improve stability. For example, U.S. Pat. No. 4,559,314 describes steaming a zeolite/binder composite at 200.degree.-500.degree. C. for at least an hour to enhance activity by raising the alpha. U.S. Pat. No. 4,522,929 describes presteaming a fresh zeolite catalyst so that the alpha activity first rises then falls to the level of the fresh unsteamed catalyst, producing a stable catalyst which may be used in xylene isomerization. U.S. Pat. No. 4,443,554 describes steaming inactive zeolites (Na ZSM-5) to increase alpha activity. U.S. Pat. No. 4,487,843 describes contacting a zeolite with steam prior to loading with a Group IIIB metal.
There has been no suggestion, however, to steam treat silicon pre-selectivated catalysts to enhance shape-selectivity. It has now been found that pre-selectivation treatment followed by steam treatment of a molecular sieve catalyst provides unexpectedly better results in shape selective hydrocarbon conversions than pre-selectivation alone or steam treatment alone. Furthermore, steaming alone has been found to be detrimental in the context of the present invention.
Accordingly, it is an object of the invention to improve selectivity in catalytic molecular sieves thereby improving shape selectivity in hydrocarbon conversion processes over the molecular sieves.